Autonomous data acquisition system and method

ABSTRACT

A submersible node and a method and system for using the node to acquire data, including seismic data is disclosed. The node incorporates a buoyancy system to provide propulsion for the node between respective landed locations by varying the buoyancy between positive and negative. A first acoustic positioning system is used to facilitate positioning of a node when landing and a second acoustic positioning system is used to facilitate a node transiting between respective target landed locations.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International Application No.PCT/AU2019/050508, filed May 23, 2019, which claims priority to AUApplication No. AU 2018901806, filed May 23, 2018, under 35 U.S.C. §119(a). Each of the above-referenced patent applications is incorporatedby reference in its entirety.

BACKGROUND OF THE INVENTION Technical Field

An autonomous data acquisition node, system and method are disclosed.The data may be marine or more particularly submarine data, includingbut not limited to one or more of seismic data and other data.

Description of the Related Technology

The genesis of the disclosed node, system and method stems fromconsidering the quality and cost of acquiring marine seismic data. Theacquisition of marine seismic data (also known as marine seismicsurveying) is a critical initial phase in the exploration of offshoreenergy or mineral reserves.

One marine seismic surveying technique involves the use of a marinesurvey vessel to tow a seismic source and sensors through a body ofwater over a survey area whilst actuating the seismic source at selectedintervals of time. The seismic source generates seismic waves whichtravel through the body of water and the subsurface generatingreflection and refractions at interfaces associated with geologicallayers and formations. The reflected waves are detected by the sensorsbeing towed by the vessel. Thus, there is a hydraulic coupling of theseismic energy from the seabed and underlying geology to the sensors.

In an alternate technique, seismic sensors are physically laid on theseabed to detect seismic energy which emanated from a controlled sourceand is subsequently reflected by the seabed and underlying geology. Inthis method there is a mechanical coupling of the seismic energypropagating through the seabed.

The mechanical coupling of sensors provides better quality data thanhydraulic coupling. However, a drawback with the mechanical coupling isthe difficulty and thus cost of placing and positioning the sensors onthe seabed and subsequently moving them multiple times to cover adesignated survey area. This is much easier and cheaper when sensorstreamers are towed behind a vessel and reside in or near the waterplane.

GB2541189 (Autonomous Robotics Limited) describes an autonomousunderwater vehicle (AUV) that can be used for recording seismic signalsduring a marine seismic survey. The AUV is described as having a passivebuoyancy system where a bladder holds a gas which is compressed byhydrostatic pressure as the AUV descends under the power of a thruster.Therefore, there is no ability to actively control or vary the buoyancyof the AUV. A propeller or thruster is used to both drive the AUV to theseafloor and also for recovery from the seafloor.

This specification is totally silent on the issues of navigation of theAUV and acoustic communication between the AUV and surface vehicles.

U.S. Pat. No. 9,417,351B2 (CGG Services SA) describes a system andmethod for conducting marine seismic surveys. This uses two clusters ofAUVs each associated with an unmanned surface vessel (USV) to conductmarine survey. The USV follow paths independent of each other. A centralcontrol vessel controls the USVs. The AUV do not land on the seabed toacquire seismic data, rather they acquire their data while travellingthrough the water. This document does mention that an AUV can be“parked” on the ocean bottom if its depth is no more than a few hundredmetres and the ocean bottom currents are not too strong. Then when a newsurvey needs to be performed the AUV may be awakened by a correspondingUSV. The awakened AUV then operated in all manner to acquire data whilein transit through the water rather than when on the seabed.

This document describes the use of an acoustic underwater positioningand navigation system installed on the USVs to communicate with theAUVs. This uses super Short Baseline (SSBL) or Ultra Short Baseline(USBL) communication. Each UV also described as optionally having aWi-Fi communication system. This is to enable data transfer between anAUV and USV, and also for the purposes of obstacle avoidance.

U.S. Pat. No. 6,951,138B1 (Jones) describes a multi component oceanbottom seismometer (MOBS) that can be deployed from a vessel and swim toa location on the seabed to acquire seismic data, then either swim backto the vessel or to a second location on the seabed. The MOBS uses finsto dig into sea bottom to improve coupling. The fins also form part of apropulsion system that may optionally also include one or morepropellers.

The MOBS a buoyancy system which essentially comprises a fixed volumeballast tank 41 they can be emptied or filled to provide a requiredbuoyancy.

The MOBSs can swim to a new survey location after a completed survey.The navigation is provided by acoustic transponders 46, 50, 52 that mustbe installed on the seabed prior to commencing a survey. While thesurface support ship 48, MOBSs and seabed acoustic transponders to anavigation system can communicate with each other this document does notdescribe any particular method of acoustic communications.

US2013/0083624A1 (CGGVERITAS Services SA) describes a system and methodfor conducting marine seismic surveys similar to that in U.S. Pat. No.9,417,351B2, except that the AUVs navigate to the seabed where theyacquire seismic data. The buoyancy system 134 and this document has anumber of chambers that can be flooded with water to modify the buoyancyof the AUV. Is further disclosed that an accumulative holding compressedgas can be provided on the AUV to expel water from the chambers. Thecommunications system is very rudimentary and limited, essentiallymimicking that in U.S. Pat. No. 9,417,351B2.

US2017/0137098A1 (Seabed Geosolutions BV) describes an AUV for acquiringseismic data from the seabed. A buoyancy control system 134 is providedfor controlling the AUV depth. The ability to vary the buoyancy isdescribed at [0035] by releasing a degradable weight on the bottom ofthe ocean. The possibility of one or buoyancy tanks that can be floodedwith air or water to assist in vertical navigation is also described.

The AUVs in this specification can communicate with other subsea devicesor a surface vessel using USBL, SSBL or SBL (short base length) systems.

Any discussion of the background art throughout this specificationshould in no way be considered as an admission that such background artis prior art, nor that such background art is widely known or forms partof the common general knowledge in the field in Australia or worldwide.

While the above background is cast in relation to marine seismicsurveying, embodiments of the disclosed method, system and nodes are notlimited in application to acquisition of seismic data and mayalternately or additionally sense or measure other characteristics orphysical attributes including but are not limited to: water temperature,density, chemical characteristics such as salinity, pH, oxygen content,carbon dioxide content, phosphate content; sulphur content;oceanographic conditions including ocean current velocity andhydrostatic pressure; gravitational field strength; magnetic fieldstrength and orientation; gamma radiation; acoustic characteristics;optical characteristics; bathymetry; and aspects of the benthic zone.

SUMMARY

In one aspect there is disclosed a submersible autonomous dataacquisition node comprising:

-   -   buoyancy system enabling variation of the node buoyancy between        positive buoyancy and negative buoyancy, the buoyancy system        including at least one inflatable external bladder and at least        one internal reservoir containing a liquid wherein the liquid        can be transferred between the at least one inflatable external        bladder and the internal reservoir to vary node buoyancy by        changing total displacement of the node.

In one embodiment the buoyancy system is operable as a buoyancypropulsion system enabling the node to transit between two submarinelocations, wherein the buoyancy system is arranged to vary node buoyancybetween a positive buoyancy and a negative buoyancy to maintain the nodewithin a transit envelope below the water surface and above a seabed tofacilitate the node traversing along an oscillating path from a firstsubmarine location to a second submarine location.

In one embodiment the submarine locations are landed locations on theseabed and wherein the buoyancy system is operable upon the node landingat a seabed location to increase the negative buoyancy of the node tothereby increase contact pressure between the node and the seabed.

In one embodiment the node is configured when landed on the seabed inmanner wherein the negative buoyancy of the node is substantially evenlydistributed over contact locations between the node and the seabed.

In one embodiment the buoyancy system comprises a pump system having atleast one pump operable to transfer liquid from the reservoir to the atleast one inflatable external bladder to thereby increase total nodebuoyancy, and a bleed path operable to selectively allow the liquid toflow from the at least one inflatable external bladder to the reservoirby action of a difference in pressure acting on liquid in the reservoirand in the at least one inflatable external bladder, to thereby decreasetotal node buoyancy.

In one embodiment the reservoir includes a pneumatic region separatedfrom a hydraulic region containing the liquid, and wherein the pneumaticregion is at a negative pressure with respect to atmospheric orhydrostatic pressure acting on the at least one inflatable externalbladder.

In one embodiment the bleed path includes a one-way valve operable toenable fluid to flow only in a direction from the at least one externalinflatable bladder to the reservoir, the valve being switchable betweenan opened state wherein liquid is able to flow from the at least oneexternal inflatable bladder to the reservoir, and a closed state inwhich the liquid is blocked from flowing through the bleed path.

In one embodiment the pump system comprises a single pump capable ofpumping liquid from the reservoir to the at least one inflatableexternal bladder.

In one embodiment the pump system comprises a transit pump and alift-off pump, wherein the transit pump is arranged to pump liquidbetween the reservoir and the bladder at a first flow rate and firstpressure, and the lander buoyancy system comprises a lander pump whichpumps liquid between the reservoir and the bladder and a second flowrate and second pressure wherein the first flow rate is higher than thesecond flow rate and the second pressure is higher than the firstpressure.

In one embodiment the node is selectively operable to broadcast acousticcommunications packets to facilitate; and capable of receiving andprocessing acoustic communications packets to facilitate one-way traveltime positioning when the node is repositioning between two submarinelocations.

In one embodiment the node comprises a top mounted transducer forbroadcasting the acoustic communications packets and a downward facingreceiver for receiving acoustic communications packets transmitted byanother node.

In one embodiment the node is arranged to communicate with a surfacemarine vessel using USBL, SBL or SSBL acoustic communications.

In one embodiment the node comprises a surface communications systemenabling a node to communication when on a water surface with the oranother surface vessel and/or a land-based station.

In one embodiment the node comprises a GPS receiver.

In one embodiment the node comprises a thruster arranged toautomatically operate when speed of a node derived from operation of thebuoyancy propulsion system is less than a threshold speed.

In a second aspect there is disclosed an autonomous seismic dataacquisition node comprising the node according to the first aspect andone or more seismic sensors supported on the node for acquiring seismicdata.

In one embodiment the node comprises one or more other sensors.

In a third aspect there is disclosed an autonomous ocean dataacquisition node comprising the node according to the first aspect andone or more sensors capable of sensing one or more oceanographiccharacteristics or properties.

In a fourth aspect there is disclosed marine data acquisition systemcomprising a plurality of nodes according to the first aspect.

In a fifth aspect there is disclosed a marine seismic data acquisitionsystem comprising:

-   -   one or more surface vessels,    -   a plurality of autonomous nodes capable of acquiring seismic        data at one or more seabed locations, each node having a        buoyancy propulsion system enabling each node to reposition        between respective landed seabed locations without surfacing by        variation of the node buoyancy between positive buoyancy and        negative buoyancy;    -   a first acoustic positioning system operable between one of the        surface vessels and the nodes; and    -   a second acoustic positioning system operable between respective        submerged nodes.

In one embodiment the first acoustic positioning system is a USBL, SSBLor SBL system.

In one embodiment the second acoustic positioning system comprises aone-way travel time positioning system.

In one embodiment the one or more surface vessels comprise a nodemothership having a command and control system arranged to providecentralised communication, control and monitoring of the nodes and othersurface vessels.

In one embodiment the system comprises a wireless mesh communicationsnetwork for enabling communication between the command and controlsystem and the nodes via acoustic modems associated with at least thefirst acoustic positioning system.

In one embodiment the system comprises an acoustic doppler currentprofiler installed on the node mothership for providing real time watercurrent velocity profile data to the command and control system.

In one embodiment the command control system is arranged to process andutilise the real-time current profile data to exert navigation controland scheduling of the nodes.

In one embodiment the system comprises a multibeam echo sounderinstalled on the node mothership and arranged acquire and providebathymetric information in real-time to the command and control system.

In one embodiment the system comprises one or more containerised nodedocking systems located on a deck of node mothership wherein eachcontainerised node docking system comprises a container having aplurality of docks, wherein each dock is arranged to house acorresponding node.

In one embodiment each containerised node docking system comprises apower distribution system arranged for electrically charging nodes whenin their respective docks.

In one embodiment each containerised node docking system comprises acommunications network arranged to enable communication and datatransfer between a node in its dock and the command and control system.

In one embodiment each containerised node docking system comprises anautomated handling system arranged to transfer a node into and out of adock.

In one embodiment the node mothership comprises a launch and recoverysystem arranged to transfer a node from a first of the containerisednode docking systems to a launch location on the node mothership, andtransfer a node from a recovery location on the node mothership to thefirst or another one of the containerised node docking system.

In a sixth aspect there is disclosed a method of conducting a marineseismic survey comprising:

-   -   transmitting seismic energy from a seismic source vessel;    -   autonomously landing a plurality of nodes on a seabed at        respective first locations to form an array of nodes;    -   using the landed nodes to acquire seismic data at the first        locations;    -   varying the buoyancy of at least some of the nodes between a        positive buoyancy and a negative buoyancy to facilitate transit        of the nodes from their respective first locations to respective        second landed seabed locations without the nodes surfacing; and    -   using the landed nodes to acquire seismic data at the second        locations.

In one embodiment varying the buoyancy between positive and negativebuoyancy comprises controlling flow of a liquid between an inflatablebladder external of a body of the node and a reservoir internal of thebody of the node.

In one embodiment the controlling flow of the liquid comprises:utilising a pressure differential to allow a liquid to bleed from thebladder to the reservoir to reduce a total displacement of the nodethereby reducing the buoyancy of the node; and pumping liquid from thereservoir to the bladder to increase the total displacement of the node.

In one embodiment utilising a first acoustic positioning system operablebetween one or more surface vessels and the nodes to position the nodesat their respective first and second landed locations; and

-   -   utilising a second acoustic positioning system operable between        respective submerged nodes to navigate the transiting nodes from        their first locations to their second locations.

In one embodiment the first acoustic positioning system comprises usingUSBL, SSBL, or SBL acoustic positioning.

In one embodiment the method comprises configuring respective groups ofthe nodes to have a same USBL, SSBL, or SBL beacon address andactivating nodes in each group in a manner wherein no two nodes with thesame USBL, SSBL, or SBL beacon address actively communicate via anassociated USBL, SSBL, or SBL modem at the same time.

In one embodiment the method comprises utilising a second acousticpositioning system comprises using a one-way travel time acousticpositioning.

In one embodiment the method comprises operating a set of nodes at theirrespective first landed locations to operate as one-way travel timebeacons and broadcast acoustic communication packets.

In one embodiment the method comprises enabling the transiting nodes toreceive and use the broadcast an acoustic communication packets tonavigate from their first landed location to a second landed location.

In a seventh aspect there is disclosed a method of conducting a marineseismic survey comprising:

-   -   transmitting seismic energy from a seismic source vessel;    -   positioning a plurality of nodes having seismic sensors to        respective first landed locations on a seabed using a first        acoustic positioning system;    -   acquiring seismic data at the respective first landed locations        using the seismic sensors of the landed nodes;    -   transiting at least some of the nodes from their respective        first landed locations toward respective second landed locations        using a second acoustic positioning system operable between a        set of the landed nodes and transiting nodes; and    -   positioning the at least some of the nodes at their respective        second landed position utilising the first acoustic positioning        system.

In one embodiment using a first acoustic positioning system comprisesusing USBL, SSBL, or SBL acoustic positioning.

In one embodiment the method comprises configuring respective groups ofthe nodes to have a same USBL, SSBL, or SBL beacon address andactivating the nodes in each group in a manner wherein no two nodes inthat group with the same USBL, SSBL, or SBL beacon address activelycommunicate via an associated USBL, SSBL, or SBL modem at the same time.

In one embodiment utilising a second acoustic positioning systemcomprises using a one-way travel time acoustic positioning.

In one embodiment the method comprises activating a set of nodes attheir respective first landed locations to operate as one-way traveltime beacons and broadcast acoustic communication packets.

In one embodiment the method comprises enabling the transiting nodes toreceive and use the broadcast an acoustic communication packets tonavigate from their first landed location to a second landed location.

In one embodiment the method comprises acquiring bathymetric data duringthe survey in real-time and modifying an associated survey plan on thebasis of the acquired bathymetric data.

In an eight aspect there is disclosed a method of remotely conducting anocean survey comprising:

-   -   varying buoyancy of one or more ocean data acquisition        submersible between a positive buoyancy and a negative buoyancy        to facilitate transit of the one or more nodes from a deployment        location to a first submersed survey location, wherein each node        includes one or more ocean data sensors;    -   while transiting, enabling one or more nodes to autonomously        navigate from the deployment location to the first submersed        survey location; and    -   using the one or more ocean data sensors when at the first        survey location to acquire ocean data.

In one embodiment the method comprises autonomously landing the one ormore nodes on a seabed at the first survey location wherein the oceandata is acquired while the one or more nodes are landed on the seabed.

In one embodiment the method comprises triggering the one or more nodeslanded on the seabed to transit for reposition to a second surveylocation wherein the transit is performed, or assisted, by variation ofthe buoyancy of the node.

In one embodiment the triggering is performed on the basis of: aneffluxion of time; or, in response to information collected by the oneor more ocean data sensors.

In one embodiment the method comprises causing a node to surface duringtransit from the first survey location to the second survey locationand, while surfaced, enabling the node to (a) acquire GPS data, or (b)transfer sensed ocean data to another node, or (c) wirelessly transfersensed ocean data to a remote location, or any two or more of (a), (b)and (c).

In one embodiment the method comprises transporting the one or morenodes to the deployment location on a manned or an unmanned surfacevessel.

In a ninth aspect there is disclosed a method of remotely conducting anocean survey comprising:

-   -   transiting one or more nodes accordance with the first aspect        from a deployment location to a first submersed survey location,        wherein each node includes one or more ocean data sensors;    -   while transiting, enabling one or more nodes to autonomously        navigate from the deployment location to the first submersed        survey location; and    -   using the one or more ocean data sensors when at the first        survey location to acquire ocean data.

In one embodiment the first survey location is a landed location on aseabed.

In one embodiment the ocean data includes water temperature, density,chemical characteristics such as salinity, pH, oxygen content, carbondioxide content, phosphate content; sulphur content; oceanographicconditions including ocean current velocity and hydrostatic pressure;gravitational field strength; magnetic field strength and orientation;gamma radiation; acoustic characteristics; optical characteristics;bathymetry; and aspects of the benthic zone.

BRIEF DESCRIPTION OF THE DRAWINGS

Notwithstanding any other forms which may fall within the scope of thesystem and method as set forth in the Summary, specific embodiments willnow be described by way of example only with reference to theaccompanying drawings in which:

FIG. 1a is a schematic representation in plan view of a first embodimentof the disclosed system and method for acquiring data;

FIG. 1b is a schematic representation of a rolling nature of the systemand method of acquiring seismic data of the first embodiment;

FIG. 2 illustrates how the first embodiment of the disclosed system andmethod enable the acquisition of data over an area greater than thatcovered by a plurality of autonomous data acquisition nodes which may beincorporated in the disclosed method and system for acquiring data;

FIG. 3 is a side view of one embodiment of a node that may beincorporated in various embodiments of the disclosed method and systemfor acquiring data;

FIG. 4 is front view of the node shown in FIG. 3;

FIG. 5 is an isometric view from one side of the node shown in FIG. 3;

FIG. 6a is an isometric view of a skid incorporated in the node shown inFIGS. 3 and 4;

FIG. 6b is a representation of a releasable coupling system that may beused to releasably couple a body of the node to the skid;

FIG. 7a is a schematic representation a buoyancy system incorporated inone embodiment of the disclosed node;

FIG. 7b is a graphical representation of a possible operating cycle ofthe buoyancy system shown in FIG. 7 a;

FIG. 7c is a schematic representation of a flight path of a nodetransiting from a water surface to a landed location in accordance withan embodiment of the disclosed method;

FIG. 7d is a schematic representation of a final approach of a node whenlanding at a seabed location;

FIG. 8 is block diagram of the node;

FIG. 9 is a State Diagram of a node;

FIG. 10a is a representation in isometric view of a containerised nodedocking system incorporated in embodiments of the disclosed method andsystem;

FIG. 10b is a front view of the containerised node docking system shownin FIG. 10a with the one end of an associated container being opened;

FIG. 10c is a view of section AA of the containerised node dockingsystem shown in FIG. 10 b.

FIG. 11a is a representation of a launch and recovery systemincorporated in an embodiment of the disclosed system when launchingnodes into the body of water; and

FIG. 11b is a representation of the launch and recovery system shown inFIG. 11b but now when being operated to recover nodes from the body ofwater.

FIG. 12 is flowchart depicting the first embodiment of the disclosedmethod for acquiring data;

FIG. 13 is a representation showing how nodes in the system may bedeployed and initially positioned in accordance with steps in the firstembodiment of the disclosed method;

FIG. 14 is a plan view of an embodiment of the disclosed system at thecommencement of performing an embodiment of the disclosed method showingthe nodes in a landed array and possible starting locations for othermarine vessel is used in the system and method;

FIG. 15 is a plan view representing how nodes in the system may berepositioning in accordance with steps in the first embodiment of thedisclosed method;

FIG. 16 is a front view of the nodes being repositioned as shown in FIG.14;

FIG. 17 is a plan view representing how nodes in the system may berecovered from the body of water in accordance with steps in the firstembodiment of the disclosed method;

FIG. 18a is a schematic representation of a route of a node operated inaccordance with the second embodiment of the disclosed system andmethod; and

FIG. 18b is a graphical representation of a possible operating cycle ofthe buoyancy system of the node operated in accordance with the secondembodiment.

DETAILED DESCRIPTION OF CERTAIN INVENTIVE EMBODIMENTS

The following is a nonlimiting description of embodiments of anautonomous data acquisition system and method as well as an autonomousnode that facilitate the system and method. The autonomous nodes carrysensors for sensing or measuring and collecting data pertaining tocharacteristics or physical attributes including but not limited to:seismic data; water temperature, density, chemical characteristics suchas salinity, pH, oxygen content, carbon dioxide content, phosphatecontent; sulphur content; oceanographic conditions including oceancurrent velocity and hydrostatic pressure; gravitational field strength;magnetic field strength and orientation; gamma radiation; acousticcharacteristics; optical characteristics; bathymetry; and aspects of thebenthic zone. In this way a node can be considered to be a sensordelivery system.

First Embodiment—Seismic Survey Application

A first embodiment the disclosed node, system and method is described inrelation to the data being seismic data derived from energy reflectedfrom a surface of or otherwise propagated through a seabed. The nodesmay be arranged in an array having an array footprint area. Embodimentsof the disclosed system and method enable the continuous acquisition ofseismic data over an area many times the array footprint area. This ispossible because the nodes can autonomously reposition from respectivefirst locations to second locations so that the array progressivelytraverses across the seabed. This provides the self-locating nature ofthe nodes. Thus, the nodes can form a rolling array that canprogressively and continuously roll or traverse across or over thesurvey area, which may be many times larger than the array footprintarea.

General System and Method Characteristics

The method 10 and system 100 are very well suited to marine seismicsurveying. Embodiments for marine surveying allow a survey vessel tooperate continuously even in deep water (e.g. exceeding 1000 m depth)without the need to wait for repositioning or moving of seismicdetecting nodes because the nodes reposition by themselves. In suchembodiments the method and system also include use of a seismic source.A subset of the nodes which are outside of the shot offset areareposition themselves to their second location on the seabed. Thesubsets of nodes may be triggered to reposition upon (a) passage of time(i.e. a timer); or (b) receipt of a remotely transmitted signal; or both(a) and (b). Other nodes in the array remain in their first locationsactively acquiring seismic data until they are triggered to repositionto their second locations.

Initial modelling has indicated that embodiments of the disclosedmethod, system and nodes when applied to the marine survey environment,can provide substantial time saving that may provide a cost saving inthe order of 70% for a deep water seismic survey.

FIGS. 1a, 1b and 2 depict in a general sense how nodes N in anembodiment of the disclosed data acquisition system 10 move/repositionto successive seabed locations to conduct a marine seismic survey over awide area. The system 10 comprises a plurality of autonomousself-locating nodes N_(xy) (hereinafter referred to in general as “nodesN”). Each node N is provided with sensors for sensing physical events orcharacteristics such as seismic energy which is reflected by orotherwise propagated through a seabed. The seismic energy may beinitially transmitted via a seismic source 12. As is understood in theart the seismic energy is reflected from a surface or at a boundarybetween materials of different refractive index in the seabed. Each ofthe nodes receives seismic energy in a specific location on the seabed.This may be by way of the equipping the nodes N with geophones,hydrophones, accelerometers or other seismic sensors.

In the embodiment shown in FIG. 1a the nodes N are arranged in a 4×4array on the seabed. A 4×4 array is used for illustrative purposes only.In practise embodiments of the system 10 and method may use severalhundred or thousand for example, but not limited to between 500-10,000autonomous nodes N. This number of nodes may provide an array area ofbetween 20 Km²−400 Km².

The nodes in the array may be designated as N_(xy)a where x and y denoteX and Y location coordinates respectively for a node N with reference toa Cartesian coordinate system projected onto an array at seabed location“a”. For example, the node N_(11a) is the node at x,y location 1,1 in anarray at location “a”: node N_(23a) is the node at x,y location 2,3 inthe array location “a”.

In the system 10 each of the nodes N is arranged to autonomously move toa second respective seabed location “b”. At their location “b” the nodesN are also able to receive seismic energy reflected or otherwisepropagated through the seabed S.

In a marine seismic embodiment or application of the system 10 appliedto the marine environment the seismic moving seismic source 12 may be onor towed by marine vessel.

Looking again at the embodiment of system 10 shown in FIG. 1a in thisexample the 4×4 array of nodes N can acquire seismic data for a surveyarea A11. As a seismic survey is being conducted using the system 10 andcorresponding method, the nodes N progressively move from their firstlocation “a” to their second location “b”. The autonomous movement ofthe nodes N from their first location “a” to their second location ‘b”is in response to a trigger signal. The trigger signal can be based onthe passage of time and thus be generated by a local or remote timer. Inanother embodiment the trigger signal may be a non-temporal signaltransmitted from a remote location, for example form an unmanned surfacevehicle. The remote location may be the same location as the source 12.

While some of the nodes N are in transit moving from their location “a”to “b” other nodes N are landed on the seabed and receiving or otherwiseacquiring seismic data. Eventually however all the nodes N willautonomously move/reposition from their first location “a” to theirsecond location “b”. In one example, if the source 12 is traversing backand forth in a direction parallel to say the Y axis of the arrays, thenthe nodes N progressively move in a sequence of X row 1, then row 2, row3 and row 4.

In more detail the node N_(11a) may reposition to location “b” and thusbecome designated as node N_(11b). Thereafter the nodes N_(xya)reposition in the following sequence to their respective location “b”and thereafter are designated as nodes N_(xyb).

N_(12a)→N_(12b)

N_(13a)→N_(13b)

N_(14a)→N_(14b)

N_(24a)→N_(24b)

N_(23a)→N_(23b)

N_(43a)→N_(43b)

N_(44a)→N_(44b)

The nodes N_(11a), N_(12a), N_(13a), N_(14a) in the first x-axis row inthe area A11 which have moved/repositioned to their second row atlocation “b” now become nodes N_(11b), N_(12b), N_(13b), N_(14b). Thefirst x-axis row of the 4×4 array has moved four rows to the right, overthe remaining nodes in area A11. This is illustrated in FIG. 1b whicheffectively shows the 4×4 array translated “one row” to right and intoarea A11′. In this example the footprint area of the array of nodes Nhas not changed but it's location has translated to the right. In thisway the array of nodes can traverse an area many times that of itsfootprint area. When the second x-axis row of nodes at location “a” haverepositioned to their locations “b”, they have moved four rows to theright to area A11″ and so on. As this continues for all the nodes, thearray of nodes N effectively rolls along the seabed enabling theacquisition of seismic data for an area of seabed many times that of thearray measurement area.

When all the nodes in the array at location “a” have repositioned totheir respective location “b” then the node's footprint area is thatdesignated as A21 in FIG. 1a . This is the same as area A11, buttranslated by a length equal to the transverse length of the area A11.Therefore, the same array of nodes N can now provide seismic survey datafor the area A21.

The operation and motion of the nodes at location “b” continues in thesame way as described above, with the nodes N_(xyb) acquiring seismicdata pertaining to the seabed S for the measurement area A12. The nodesN_(xyb) will progressively reposition to location “c”. (The locations“b” and “c” can be considered as new first and second locationsrespectively).

In some embodiments the effective measurement array of nodes N, that isthe nodes which are active at any one time acquiring seismic data willinclude a combination of nodes at locations “a” and “b”. For example,with reference to FIG. 1a after the nodes in the first x-axis row in thearray area A11 have moved to their location “b” on the seabed, they maybe activated to acquire seismic data together with say the nodes in thethird and fourth x-axis row of the of the array at location “a”. Inother words, the system and method may enable acquisition of seismicdata from a substantially constant number of nodes N covering asubstantially constant (though moving) area.

The above process continues until the entire survey area is surveyed.The nodes, and therefore the measurement area A, can be steered in anydesired manner to progressively cover the survey area. This isexemplified in FIG. 2 which shows the measurement area A firsttraversing to the right from A11 to Am1, then north or up to area Am2,progressively to the left to area A12 and so on until the lastmeasurement area Amn is surveyed. This can be performed continuously asthe source 12 traverses across the seabed survey area.

Nodes

FIGS. 3-8 depict an embodiment of the node N when the system 10 andassociated method is used for marine seismic surveying. In thisembodiment the node N is a submersible node having a body 14 with agenerally hydrodynamic shape. The body 14 includes a high-pressure hull,which may be made from aluminium with machined aluminium end caps.

Major external, or at least partially external aspects of each node N,include a buoyancy system 16, a thruster 18, a skid 20 and controlsurfaces/fins 22.

The body 14 in this embodiment is releasably coupled to the skid 20which supports the body 14 when landed on the seabed. The skid 20provides a stable base which engages with the seabed. The body 14remains in a fixed relative disposition to the skid 20. In this way theorientation of the body 14 on the seabed is relatively predictable.

In this embodiment the skid 20 is formed with a generally rectangularbase frame 13 with an upwardly kicked front-end 15. Posts 17 are affixedto the base frame 13 and space the body 14 from the skid 20. Areleasable coupling system 19 releasably couples the body 14 to the skid20. The releasable coupling system 19 may be achieved by many differentmechanisms including but not limited to a burn wire, a screw threadcoupling, or a bayonet connection. In this illustrated embodiment thereleasable coupling system 19 is a burn wire system which includes asacrificial rope 21 that ties a lug 23 fixed to the skid 20 to a hullclamp 25 fixed to the body 14. The releasable coupling system 19 has aburn wire assembly 27 connected to opposite ends of the sacrificial rope21. Activation of the releasable coupling system 19 enables the body 14of the node N to be retrieved in the event of the skid 20 is detected asbeing caught on a seabed structure; or otherwise stuck on the seabedfollowing being commanded to take-off.

In an alternate embodiment the skid 20 which orientates the body 14 on,and provides mechanical coupling with, the seabed, can be replaced withother structures that perform the same functionality. One example ofthis is a fairing 20′ which may be provided with three or more contactpoints or regions that orientate the node N on the seabed and providemechanical coupling. It will be understood that the provision of forexample three contact points will orientate the body 14 in a plain wherea central longitudinal axis of the body 14 is a known orientationrelative to that plain.

Irrespective of the nature and configuration of the skid, fairing orindeed the body 14 itself, the node is configured in manner so that whenlanded on the seabed, the negative buoyancy of the node is substantiallyevenly distributed over contact locations between the node and theseabed. For example, when the node N is provided with the skid 20 thecontact locations would ordinarily be along the base frame 13. However,if a fairing 20′ is used instead of the skid the contact locations wouldbe the contact points or regions of the fairing that are designed tocontact the seabed and correctly orientate the node N/body 14. In yet afurther variation a node may include both a skid 20 and a fairing 20′.

In this alternate arrangement the option of automatically releasing fromthe skid 20 is not available. However, recovery assistance can berealised by jettisoning the ballast/weight 82 (shown in FIG. 8 anddescribed later) to provide additional buoyancy to allow the node tosurface, even in the case of buoyancy system failure.

The weight of the skid 20 can be matched to the available buoyancy rangeof the node so that dropping the skid will result in the node having apositive buoyancy but where the buoyancy can still be modulated betweenpositive and negative buoyancy to maintain the functionality of thebuoyancy propulsion system (discussed further below). In this way thenode is still able to move to a recovery location, propelled at least inpart by modulation of buoyancy.

The node N may also be provided with a downward facing acoustic receiver11 for receiving acoustic communication packets from other nodes whichare operated or otherwise activated to act as position referencebeacons. The nodes which are capable of operating as beacons may beprovided with a top mounted transducer for broadcasting the acousticcommunications packets. This aspect is described later in thespecification.

The buoyancy system 16 provides buoyancy for the purposes of (a)transiting (i.e. propulsion) of the node through the water (b) landingand recovery of the node, and (c) control seabed coupling. The buoyancysystem 16 operates to vary the displacement of the node by varying theoverall volume of water displaced by the node, i.e. the totaldisplacement of the node. This is to be contrast with buoyancy systemsthat operate by moving a liquid and gas into or out of fixed volumechambers.

The buoyancy system 16 has at least one internal fluid reservoir 26(which may be of fixed or variable or flexible volume), a pump system28, and at least one (two shown in FIG. 8) external inflatable (i.e.variable volume) bladder 30. In this context the term “external” meansthat the inflatable bladders 30 are in fluid communication with ambienthydrostatic pressure; and their variation in volume results in anoverall variation of the volume (i.e. displacement) of the node. Forexample, this would exclude an inflatable bladder located within body14, that is in fluid communication with ambient hydrostatic pressure(for example via a hole in the body 14) but where a change in volume ofthe inflatable bladder does not change the overall volume of the node.

The location of reservoir 26 and the bladder 30 may be arranged in a wayso that when a node is landed on the seabed, its landed negativebuoyancy is distributed evenly over the skid 20 or seabed contact pointsof the node to assist in providing stable coupling with the seabed. Inembodiments where a node has more than one internal reservoir 26 theinternal reservoirs may be in fluid communication with each other. Thisthen allows control of the liquid distribution within the multipleinternal reservoirs. In turn this provides control of the massdistribution of the node.

In one example this may be arranged by attaching the bladder(s) 30 andoutside of the body 14. A fairing (shown in phantom as item 20′ in FIG.8) may also be coupled to the body 14 (in addition, or as analternative, to the skid 20) about the bladder(s) 30 to provideprotection from damage and improved hydrodynamic profile. For ease ofdescription the embodiment will be described hereinafter as having onebladder 30 external of the body 14. By operation of the pump system 28an incompressible fluid, i.e. a liquid such as an oil, can be movedbetween the reservoir 26 within the body 14 and the bladder 30 externalto the body 14. The bladder 30 is inflated by the fluid causing anincrease in the displacement (i.e. volume) of the bladder 30 and thus anincrease in the buoyancy of the node N.

The bladder 30 can be inflated with the fluid to provide a positivebuoyancy for the node N during flight of the node N, i.e. as it liftsfrom the seabed and rises to its transit depth. To cause the node N todive the fluid is bled from the bladder 30 back to the internalreservoir 26 thereby reducing the displaced volume of the node N toprovide a negative buoyancy. As explained in greater detail below thebleeding of the fluid back to the internal reservoir 26 is by way ofpressure differential between the reservoir 26 and the bladder 30.

Once the node N has landed on the seabed the buoyancy system may beoperated to control seabed coupling, i.e. the contact pressure betweenthe node N and the seabed. This enables control of the degree of sinkingof the node N into the seabed and the degree of coupling between theseabed and the node N. The negative buoyancy can additionally oralternately be increased or otherwise controlled for the purpose ofholding the node N stationary at its landed location. For example, thenegative buoyancy can be arranged to be sufficient to hold the node Nstationary on the seabed in currents of a predetermined velocity.Maintaining the node N stationary while actively acquiring seismic dataassists in terms of data quality.

FIG. 7a is a schematic representation of the buoyancy system 16 showingthe internal reservoir 26, the pump system 28, and the bladder 30. Thereservoir 26 has a cylinder 35, and an internal sliding piston 37. Thepiston 37 divides the cylinder into a hydraulic region 39 containing oiland a pneumatic region 41 which may be evacuated or at a negativepressure relative to at least ambient pressure. The regions 39 and 41are of variable volume and separate from each other by the piston 37. Ableed path 43 enables fluid communication between the bladder 30 to flowback to the reservoir 26. The fluid communication is controlled by aone-way valve 45 in the bleed path 43. When the valve 45 is open liquidis able to flow in the direction from the bladder 30 to the reservoir26. When the valve 45 is closed no liquid flow is possible through thebleed path 43.

When the valve 45 is open the liquid flow is by virtue of pressuredifferential between the reservoir 26 and the bladder 30. The pressuredifferential is the differential between hydrostatic pressure operatingon the bladder 30 and the vacuum or release relative vacuum in theregion 41 of the reservoir 26. In this way the bleed of liquid occurswithout the need to activate any pumping mechanism so that the onlypower required is to open the valve 45. This assists in reducing powerconsumption and extending the endurance of the node.

The pump system 28 may be in the form of a single pump, or asillustrated in the present embodiment two separate pumps, namely atransit pump 47T and a lift-off pump 47L.

Irrespective of the number of pumps included in the pump system 28, thepump system operates to transfer fluid (i.e. liquid) from the reservoir26 to the bladder 30 and thereby increase buoyancy. The bleed path 43when opened returns liquid from the bladder 30 to the reservoir 26 tothereby decrease buoyancy. When a node has landed on the seabed, byallowing this flow through the bleed path 43 to continue, the resultingadditional negative buoyancy increases the coupling force between thenode and the seabed.

In the current illustrated embodiment where the pumping system 28includes a separate transit pump 47T and lift-off pump 47L, the pump 47Tmay be in the form of a relatively high flow rate low pressure pumpwhile the lift-off pump 47L may be in the form of a relatively low flowrate higher pressure. In the alternate embodiment a single pump can beoperated to provide the same effect as the two separate pumps. However,in the dual pump embodiment using transit pump 47T during transit phasesmay provide increased node endurance and improve buoyancy propulsioneffectiveness due to faster, more efficient pumping during inflections.

The operation of the buoyancy system 16 for the purposes of transiting anode through the water, facilitating the landing and recovery of a node,and controlling seabed coupling is described in relation to FIGS. 7b, 7cand 7d . FIG. 7b illustrates the operation of the pump system 28. FIG.7c shows the corresponding effects on the node behaviour in the water.FIG. 7d shows a possible node touch down flight path.

In FIG. 7b a neutral buoyancy level is designated by the number “0” onthe vertical buoyancy axis. The neutral buoyancy is the buoyancy ordisplacement of the node required for no vertical movement, i.e. whenthe density of the node matches the density of the water.

In FIG. 7c the vertical depth axis shows a level 0 being at the watersurface, and a depth B representing the depth of the seabed from thewater surface. A transit envelope TE is represented on FIG. 7c as beingbetween an upper transit depth TU, which is below the water surface, anda lower transit depth TL which is below the upper transit depth TE butabove the depth B of the seabed.

When a node is initially launched it may be configured to have apositive buoyancy to float on water surface at the depth of zero. Inthis configuration the bladder 30 is at least partially but more likelyfully inflated with liquid thereby expanding its volume to provide thebuoyancy required for the node to float. In order for the node to reacha first target landed location L1 (FIG. 7c ) the buoyancy system 16 isoperated to decrease the displacement of the node by opening the valve45 to allow liquid to flow from the bladder 30 to the reservoir 26through the bleed path 43 by way of pressure differential. This occursat time t1 shown on FIG. 7 b.

As a result of this as shown in FIG. 7c the node sinks or dives. Thefluid transfer is controlled so that the node sinks to the lower levelTL of the transit envelope TE at a specified vertical velocity. Thevertical velocity (i.e. for diving or rising) can be derived from therate of change of pressure. This is measured by an onboard pressuresensor 70 (shown in FIG. 8). If the node is diving too quickly the pumpsystem 28 is operated to increase buoyancy, if the node is diving tooslowly the bleed path 43 is opened to reduce buoyancy. (The converseapplies when the node is rising, i.e. rising too quickly the bleed path43 is opened, rising too slowly the pump system 28 is operated.) Whenthe vertical velocity is within a required or desired band, then thenode buoyancy is adequate for the corresponding transit phase and thebuoyancy system 16 ceases to transfer liquid between the bladder 30 andthe reservoir 26. Thus, power is only consumed by the buoyancy system 16for the node propulsion to when it is required to change node velocity(i.e. node vertical speed or vertical direction). In FIG. 7c it isassumed that the start position of the node on the water surface is suchthat it is not possible to reach the target landing location L1 havingregard to the rate and pitch of descent in a single continuous dive pathwithin a reasonable time period.

Notwithstanding that the node N is provided with the thruster 18 thegeneral idea in the method of operation is to utilise the buoyancysystem for propulsion to provide increased efficiency, low powerconsumption and long duration. As a consequence, it is possible thatnodes can continue to operate autonomously and reposition betweenmultiple locations over a period of the many months (for example up to 3months or more) required to complete a seismic survey. Consequently, therequirement for recovery and recharge mid-survey can be eliminated or atleast minimised to reduce operations complexity.

When the node reaches the lower level TL of the transit envelope TE atthe time t2 the buoyancy system 16 is operated to now increase thebuoyancy of the node. This is achieved by closing the valve 45 andoperating the pump system 28 (in this case the transit pump 47T) totransfer fluid from the reservoir 26 to the bladder 30 inflating thebladder 30 thereby increasing its volume and thus the displacement ofthe node. Consequently, the node commences to rise at a trajectorycontrolled by the control surfaces 22 and a pitch controller (describedlater). During this rise the node N traverses over the seabed reducingthe lateral distance to the target landing location L1. At the time t3when the node N reaches the upper level TU the buoyancy system 16,operates to reduce the buoyancy to cause the node N to dive toward theseabed. This is achieved by stopping the operation of the pumping system28 (in this embodiment the pump 47T) and opening the valve 45 allowingoil to flow through the bleed path 43 from the bladder 30 to thereservoir 26.

The node continues to travel through the transit envelope TE oscillatingbetween the levels TU and TL in the manner described above until it iswithin reach of the landed location L1 in a single dive. In FIGS. 7b and7c , this occurs at the time t5 when the buoyancy system 16 operates tostop the pumping system 28 and open the valve 45 to provide a negativebuoyancy. This causes the node to dive below the lower level TL towardthe target landed seabed location L1. The pitch angle of the node duringthis dive is negative with respect to the horizontal.

As shown in FIG. 7d the buoyancy and pitch is controlled so that at atime t6-Δ, the node has levelled off (i.e. the longitudinal axis of thebody 14 being substantially parallel with the seabed B) andsubstantially stationary a short distance (for example up to 2 m) abovethe bottom of the seabed B where after the negative buoyancy eventuallysinks the node onto the seabed at the target landing location L1.

The negative buoyancy to affect the dive of the node to land on theseabed will provide a degree of coupling (i.e. contact force) with theseabed. However, the degree of coupling can be increased by operatingthe buoyancy system 16 to again open the valve 45 to increase thenegative buoyancy beyond that required to dive during the landing phase.The effect of this is to continue reducing the volume of the bladder 30,therefore reducing the buoyancy of the node and increasing the degree ofcoupling with the seabed and hence keeping the node stationary.

When it is required to recover or reposition a node from the seabed thebuoyancy system operates the pump system 28 to pump fluid (liquid) fromthe reservoir 26 to the bladder 30, to increase the overall displacementof the node N and thus its buoyancy.

In the present embodiment where the pumping system 28 has separatetransit and lift-off pumps, the lift-off pump 47L is initially operated(at time t7 on FIG. 7b ) to return liquid to the bladder 30 to reducethe seabed coupling and restore the buoyancy the negative buoyancy levelrequired to initially required to land on the seabed and thereby assistin the lift off of the node. Thereafter at t8 the transit pump 47T isoperated to transfer liquid to the bladder 30 to achieve a positivebuoyancy for the node to commence to reposition itself to its nexttarget landing location.

In an alternate embodiment where the pumping system 28 comprises asingle pump only then all of the above described actions of the transitpump 47T and lift-off pump 47L are performed by one and the same pump.In either case (i.e. the pump system having a single pump or dual pumps)the fluid transfer between the reservoir 26 and the bladder 30 ismonitored and the buoyancy state is set by software (in the maincontroller described later) that controls pump system 28 and bleed valve45 so that the node achieves the desired/commanded buoyancy.

When a node is traversing between successive landed/target locations thebuoyancy system 16 operates to lift the node from the seabed to thetransit envelope, and subsequently causes the node to traverse acrossthe seabed by one or more cycles of rising and diving within the transitenvelope TE until it is at a position where it can land at the nexttarget landing location in the manner described above in relation to thelocation L1.

As would be appreciated the node has now repositioned itself betweensuccessive locations along the seabed by action of the buoyancy systemalone. The thruster 18 may be operated:

-   -   during the final stages of landing if necessary in order to        achieve a particular orientation of the node on the seabed; or    -   if buoyancy propulsion speed is inadequate for system        operations—i.e. activated if the estimated node transit velocity        is below a defined threshold velocity required to reach a        desired position within a designated time period. One example of        this is may be in response to high currents which may exceed        buoyancy transit velocity.

As indicated above this provides the node N with the ability to traversebetween multiple landed locations over an extended period of time (e.g.several months) without the need to surface for mid—survey recovery andbattery recharging.

From the above description it will be apparent that the buoyancy system16 acts as and can be considered to be a buoyancy propulsion systemwhich enables or facilitates propulsion of a node between respectiveseabed locations by dynamic variation of the node buoyancy betweenpositive buoyancy and negative buoyancy.

The control surfaces/fins 22 can be autonomously (or remotely) operatedto provide control over the transit path of a node N including itslanding flight path orientation when moving to its second location “b”.

The node has a controller 32 that provides low level control of thenode. The controller 32 is operatively associated with the buoyancysystem 16, the control surfaces 22, the thruster 18 a pitch controlsystem 38. The pitch control system 38 can be operated by the controller32 to change the mass distribution in the node N. In one example thiscan be achieved for example by providing the node with a mass on athreaded shaft that extends parallel to a longitudinal axis of the body14 and then operating the shaft to move the mass fore or aft. In analternate example one or both reservoirs 26, 30 can comprise multiplefluidly connected receptacles distributed along the length of the body14 in which fluid can be selectively pumped to cause a change inbuoyancy distribution and thereby control the pitch of the node.

Power for the controller 32, and indeed all the powered systems anddevices of the node N, is provided by an on-board rechargeable batterypack 34. The battery pack 34 is coupled to a battery management system36, a power management system 39, and a battery charging connector 40accessible from the outside of the body 14. The power management system39 may include a regulator for charging the battery pack 34 andproviding regulated current and/or voltage to the node on board systemsand devices. The battery management system 36 monitors the charge of thebattery pack 34 and provides battery charge information and status tothe controller 32. The controller 32 may use this information tofacilitate an off cycle recovery of the node for the purpose ofrecharging the battery pack 34.

A main control unit 40 provides high level control and systemintegration for the node including its swarm and system behaviour. Inone example the unit 40 incorporates Mission Oriented Operating Suite(MOOS) and MOOS-IvP (Interval Programming). MOOS is an open sourcepublish-and-subscribe middleware software system for robotic platforms,and MOOS IvP is an extension to MOOS that adds a higher level ofautonomy and behaviour fusion for a robotic system.

The main control unit 40 is operatively associated with the nodenavigation and communications systems and devices. The main control unit40 is able to process inputs from the navigation system and control theoperation of the node N. The navigation system comprises a number ofdifferent systems including a GPS 50 that may optionally be augmentedwith an Iridium™ satellite communications modem 52. External antenna GPSand Iridium antennas 54 and 56 are mounted on the body 14 and connectedto the GPS 50 and the Iridium modem 52 respectively. A stable, low-driftsystem clock 58 which is coupled to the controller 32 may synchronisedvia the GPS 50 or by the CCS 98 whilst located within the CNDS 96.

To facilitate acoustic communications and navigation the node isprovided with an integrated USBL/SSBL/SBL transducer and USBL/SSBL/SBLacoustic modem 60. (Throughout this specification, except where thecontext requires otherwise due to express language or necessaryimplication, the acronym “USBL” is intended to be a reference to “USBL”,“SSBL” or “SBL”.) The USBL transducer and modem 60 facilitates: USBLacoustic communications with a surface vessel, or indeed with otherunderwater vessels including other nodes; and USBL positioning. The USBLtransducer and modem 60 may also be arranged to transmit acousticcommunications packets/beacon pings that can be used for one-way transittime (OWTT) positioning. OWTT acoustic transducers 62T and 62B aremounted on the top and bottom respectively of the node. The transducer62T is operated to broadcast acoustic communications packets, if andwhen the node is operated as an OWTT beacon. The transducer 62L is usedto receive the acoustic communications packets from the nodes acting asOWTT beacons. As described later the acoustic communications packets areused by the nodes for navigation when repositioning between successivelanded locations.

A RF modem 64 is housed within the body 14 to enable Wi-Fi or otherwireless communications. A RF antenna 66 is mounted on an outside of thebody 14 and connected to the RF modem 64 The RF modem 64 enables a nodeon the water surface to communicate with surface vessels of the systemsuch as a USV 88 or mothership 90.

Other systems and devices on the node include an inertial measurementunit (IMU) 68, pressure sensor 70 and an altimeter 72. The altimeter 72includes a transceiver 74 mounted on an exterior of the body 14. Signalsfrom the IMU 68, pressure sensor and altimeter 72 are delivered to thecontroller 32 and/or the control unit 40.

One or more sensors 76 a, 76 b (hereinafter referred to in general assensors 76) are supported by the body 14 and/or housed within the body14 for sensing various environmental conditions or events. When the nodeN is used for seismic surveying the sensors 76 a comprise seismicsensors. The seismic sensors may include geophones, hydrophones and/oraccelerometers supported by the body 14 or the skid 20. The sensors 76 areceived seismic energy either directly from the source 12, viareflection from the seabed, or after reflection/refraction of seismicenergy at boundaries within the seabed. Examples of the sensors 76 binclude those for measuring but are not limited to water temperature,density, chemical characteristics such as salinity, pH, oxygen content,carbon dioxide content, phosphate content; sulphur content;oceanographic conditions including ocean current velocity andhydrostatic pressure; gravitational field strength; magnetic fieldstrength and orientation; gamma radiation; acoustic characteristics;optical characteristics; bathymetry; and aspects of the benthic zone.The node may also be provided with still and/or video cameras.

Measurement or data from the sensors 76 is stored in an onboard datastorage 78. A data and communications connector 80 is supported on, andaccessible from the outside of, the body 14. The connector 80 allows theupload of configuration and other data and control algorithms andprograms to the node as well as the off load of sensor data stored inthe data storage 78. Additionally, data may be transmitted wirelesslyvia RF/Wi-fi or Iridium communication.

As shown in FIG. 8 of the node N may also be provided with an emergencyrecovery system 82. The emergency recovery system 82 may include aweight that can be jettisoned in an emergency situation to provide thenode with positive buoyancy to bring the node N to the water surface. Abackup locator beacon 84 that emits a signal to facilitate location ofthe node N in the case of primary system failure.

The nodes have a series of states and transitions which control theirbehaviour throughout all aspects their operation. FIG. 9 is a node statediagram and the table below briefly describes the nature of each nodestate.

On each state transition the new state of the node is logged, withtimestamp, onboard and written to a control and command system data base(CCS DB) on a node mothership (NMS) 90 (both described later withreference to FIGS. 10-18 b). Some states may immediately transition tonext state.

The node states are referred to later in the specification particularlyin relation to the description of the method 100 of acquiring data whichis described with reference to FIGS. 9-16.

TABLE 1 Node States State ID State Summary ST01 OFF Node Powered OFFST02 INITAL Node Powered ON and in initial startup state Clock 58syncronisation occurs ST03 PRE_CHECK Node Running automated systemfunctional pre- checks Results written to the command and control systemdatabase (CC DB) which is remote form the node, for example on amothership 90 ST04 PRE_CHECK_FAILED Pre-check failed - Node requiresmanual operator intervention Alert issued ST05 CONFIG Configuration &survey plan data parsed to node ST06 READY_TO_DEPLOY Node ready todeploy ST07 DEPLOYED Node in-water, on surface and positively buoyantST08 INITIAL_TRANIST Node transiting to initial commanded receiverlocation USBL_MODE set to ACTIVE (i.e. ultrashort baseline (USBL)communications and navigation/positioning equipment on the node isactive, enabling the node to receive navigation data from externalsources (such as an unmanned surface vehicle (USV) or mothership ST09TRANIST Node transiting to commanded receiver location, within commandeddepth band, using received acoustic communications packets from beaconbroadcast (for example by way of synchronous clock one-way travel time(OWTT) processing) to update dead reckoned position estimate fornavigation USBL_MODE set to SLEEP - listening on configured USBL beaconaddress (BA) but not responding or acting until received WAKEUP commandST10 LANDING Active two-way USBL comms and positioning to landinglocation & activation of landing mode ST11 LANDED Node landed andrecording data USBL_MODE set to SLEEP - listening on configured USBL BAbut not responding or acting until received WAKEUP command ST12LANDED_BEACON Landed, acting as an OWTT beacon (i.e. transmittingacoustic communications packets), periodically broadcasting preciselytime scheduled acoustic packet USBL_MODE set to SLEEP - listening onconfigured USBL BA but not responding or acting until received WAKEUPcommand ST13 TAKEOFF Active two-way USBL comms, activation and executionof take-off maneuver USBL_MODE set to ACTIVE ST14 RECOVERY Come tosurface, on surface, move to recovery congregation site ST15 RECOVEREDNode recovered and onboard vessel ST16 DOCKED Node returned tocontainerized node docking station (CNDS) which is described later inthe specification ST17 POST_CHECK Node running automated systemfunctional post- checks Results written to CC DB ST18 POST_CHECK_FAILEDPost-check failed - Node requires manual operator intervention Alertissued ST19 CHARGING_DATA Node charging Node offloading data to CC DB

FIGS. 10-18 b shown in more detail various aspects of the autonomousdata acquisition system (ADAS) 10 and associated method 100. In thepresent embodiment for the purposes of conducting a marine seismicsurvey the system 10 in addition to incorporating a large number ofnodes also includes a variety of equipment and systems that worktogether. These include the seismic source vessel 12, one or moreunmanned surface vessel (USV) 88, the node mothership (NMS) 90 and acommunications network 92 via which the surface vessels which includethe source vessel 12, NMS 90 and USVs 88 can communicate with eachother. With reference to FIG. 15, the NMS 90 has a launch and retrievalsystem (LARS) 94, one or more containerised node docking systems (CNDS)96, and a command and control system (CCS) 98.

Containerised Node Docking System (CNDS)

With reference to FIGS. 10a-10c each CNDS 96 comprises a container 200provided with a modular racking and docking system 202, and integrateddata offload and power distribution systems. The racking system 202 hasa plurality of docking bays 204, one for each node N. The powerdistribution system has battery charging apparatus available at each ofthe node docking bays 204, these charging apparatuses may include aphysical connector or wireless connection. Additionally, each CNDS 96has an integrated Wi-Fi communication system for node communication anddata offload. A high bandwidth physical ethernet connection is providedto connect the nodes N when docked in the CDNS 96 to the CCS 98 and moreparticularly it's corresponding server. The CNDSs 96 may also bearranged to enable them to be daisy-chained together for the purposes ofpower and/or communications connection.

In one example of the CNDS 96 may be based on a standard 40-foot ISOcontainer. At least two variations of the containerised node dockingsystem are possible.

In a fully automated containerised node docking system 96 illustrated inFIGS. 10a-10c the node racking system 202 has a bank of racks on eitherside internally of the containers 200. An automated node pick up andplacement system 206 is housed in a central corridor of the container200 and can run on corresponding tracks 208. The system 206 operateswhen, in a launch mode, to take a node N from its dock 204 and place iteither adjacent to or onto the launch conveyor 106. The system 206 alsooperates in a return mode in which it is able to transfer a node N fromthe return conveyor 108 into an available dock 204 in one of the CNDSs96. In some embodiments the system 206 is capable of placing a node Nonto the conveyor 106 and lifting a node N from the conveyor 108.However, in a less sophisticated embodiment the nodes can be manuallytransferred from the conveyors 106/108 onto or off of the system 206.The system 206 is controlled by the CCS 98.

In yet a further even less sophisticated embodiment the containerisednode docking system 96 may be constructed without the automated nodepick up and placement system 206 and arranged for manual transfer of thenodes N into and out of the corresponding containers 200. In such anembodiment the containers 200 may be provided with doors on one of theirsides that can be opened and sliding racks or drawers that can beaccessed when the doors are opened. The racks/drawers may be providedwith multiple docks 204 for housing corresponding nodes. With the manualCNDS each node is manually lifted from and later lifted back into adock. It is envisaged that the manual CNDS would be able to hold morenodes than the automated CNDS because no space is required for theautomated node pick up and placement system.

Launch and Retrieval System (LARS)

FIGS. 11a and 11b provide a schematic representation of a portion of adeck 102 of the NMS 90 which includes the node landing and retrievalsystem (LARS) 94 refer to earlier in this specification. FIG. 11adepicts the LARS 94 when launching the nodes N, while FIG. 11b depictsthe LARS 94 when retrieving the nodes N. These Figures also show aplurality of the CNDSs 96 and USVs 88 on the deck 102 together with acrane 104. The LARS includes a launch conveyor 106 which runs pastaccess openings or doors at one end of each CNDS 96, and a returnconveyor 108 that runs past respective access openings or doors on anopposite end of each CNDS 96. The launch conveyor 106 carries nodes N toa downwardly inclined launch chute 110 at the stern 112 of the NMS 90.The return conveyor 108 carries nodes N from a recovery chute 114 to oneof CNDSs 96.

During launch/deployment the nodes are moved onto the conveyor 106either manually or mechanically and carried to the launch chute 110 fromwhich they slide sliding down into the water. The launch process iscontrolled from the CCS 98 so that the nodes are deployed at the correctrate, in the correct sequence and the designated location.

For recovery the nodes are commanded to the surface and congregate at arecovery location, for example location 168 described later in relationto and shown in FIG. 17. From there the nodes are mechanically placed ordelivered onto the return chute 114 from which they are carried by theconveyor 108 for loading into dock in one of the CNDSs 96.

The crane 104 may be used to launch and retrieve the USVs 88.

Command and Control System (CCS)

The CCS 98 is a centralised communication, control, monitoring and datamanagement system which includes one or more servers and databases. Inthis embodiment this is provided on the node mothership 90. The CCS 98operates over the wireless mesh network 92 paired with acousticcommunication via the USVs 88. Some of the capabilities andfunctionality of the CCS 98 include:

-   -   managing scheduling of survey activities and sending commands to        the various system elements    -   allowing operators to modify survey plans mid-survey providing        flexibility in acquisition managing acquired, downloaded data        and preparing output data products for processing and analysis.

The CCS 98 includes visualisation tools such as displays enablingoperators to visualise the operation of the system 10 and location andstatus of the source 12, USVs 88 and the nodes N.

An acoustic doppler current profiler (ADCP) may be installed on the NMSto acquire real time water current velocity profile data. Collected datamay be assimilated into a local scale current velocity model within theCCS 98; or alternately used for interpolation of current velocity. Thiscan then be used in the navigation control and scheduling of the USVsand nodes, including guiding the node approach to a target landinglocation. Other surface assets (i.e. the and USVs 88) may each beinstrumented with an ADCP to provide increased area coverage. Such datamay also be used by the CCS 98 to estimate node travel time to nextlocation & evaluate flight performance of vehicles.

A multibeam echo sounder (MBES) may be installed on NMS to acquirebathymetric data. From the MBES data the CCS 98 builds a high-resolutionbathymetric model of the survey area (either from scratch or tovalidate/update past datasets). other surface assets (i.e. the and USVs88) may each be instrumented with a MBES to provide increased areacoverage. The resulting bathymetric model can be used to assist in thecontrol the node landing behaviour. In this event the altimeter 72 andcorresponding transducer 74 shown in FIG. 8 may not be required on eachnode, reducing unit cost. Additionally, collected bathymetric data maybe used to identify marine hazards such as obstacles and/or steepslopes. This data may be used by the CCS 98 to modify the survey targetlanding locations to improve reliability/results.

Acoustic Positioning

Acoustic communication and positioning imposes numerous challengesincluding its inherent limited bandwidth. Embodiments of the presentsystem 10 and method 100 seek to address this by using a combination ofdifferent acoustic positioning methods during different phases of surveyoperations. In one embodiment a first acoustic positioning system may beoperable between the surface vessels, e.g. the USVs 88 and the nodes;and a second acoustic positioning system may be operable betweenrespective submerged nodes.

For example, the first acoustic positioning system is a USBL positioningsystem. This includes USBL transceivers on the USVs 88 and the USBLtransducer/modem 60 on the nodes. The USBL positioning is used when ahigh level of positional accuracy is required, during final approach totarget location and landing, either during the initial landing and atsubsequent landings following a node repositioning. In this embodimentthe second acoustic positioning system may be in the form of an OWTTpositioning system. This entails the nodes being arranged to act as OWTTbeacons, broadcasting time scheduled acoustic communications packets viatheir top mounted transducer 62T, and also being arranged to receive theacoustic communications packets via their bottom facing transducer 62B.As previously described the OWTT may be provided by the USBLtransducer/modem 60. The landed nodes may act as OWTT beaconsbroadcasting the acoustic communication packets for the nodes transitingbetween subsequent seabed locations during mid-survey reposition events.

This contrasts with the systems described in the Background Art whichrely on USBL or LBL for all acoustic positioning. USBL is not readilyscalable and has a limited range. As a consequence, a large number ofsurface vessels are required relative to the number of nodes. The use ofLBL involves significant effort and thus cost in deploying, calibratingand recovering moored LBL transponder beacons. This is not considered tobe viable for large area coverage required for seismic acquisition.

The ability to enable synchronous clock acoustic navigation using OWTTin embodiments of the disclosed system 10 and method 100 is feasible dueto the following system attributes:

-   -   All nodes have accurate, stable, low drift clocks 58 and        synchronised time.    -   All transiting nodes are moving over, or adjacent to a landed        array of other nodes.    -   The spatial distribution of a set or group of the landed nodes        provides a suitable array for OWTT navigation.    -   The nodes do not require high positional accuracy for navigation        during transit from one side of array to the other.

This may provide several enabling factors to the overall system, whichinclude:

-   -   The degree of accuracy required from navigation systems on board        of the node may be reduced. This can provide significant cost        saving in the node components as ultra-low-cost inertial        measurement units 68 are sufficient. Additionally, the power        requirement for the low cost IMU 68 may be reduced, resulting in        increased node endurance.    -   USBL communications/navigation it is not required for the        reposition transit of the nodes—it is used for positioning        during the landing of the nodes and during recovery to provide        high accuracy positioning; and to provide communications between        surface vessels and the nodes during these periods. This reduces        the number of USVs 88 needed in the system 10. Additionally, it        becomes possible to share available USBL communication addresses        between many nodes and may lead to reduced USBL communication        usage.

As a consequence of these considerations embodiments of the system 10and method 100 may incorporate the following features:

-   -   The locations, timeslots and possibly frequency configuration of        nodes which are used as OWTT beacons may be defined for entire        survey prior to start. This information is stored onboard all        nodes prior to deployment    -   On landing at beacon location, the node is commanded to act as        beacon.    -   All node beacon locations and configurations are known by all        USV 88, the NMS 90 and other surface vehicles    -   Each beacon node broadcasts an acoustic packet on a specified        time schedule    -   Acoustic packets are received by transiting a node and used to        identify the source and calculate range from the source using        stored beacon information. Kalman filtering may be used in this        process to filter bad ranges and identify possible beacon nodes        based on speed thresholds    -   One-way time of flight measurement is made by receiving node.    -   Once a transiting node receives signals from multiple beacons        nodes it can make an estimate of its current location (latency        correction is applied using onboard dead reckoned calculations).    -   Filtering onboard the transiting node for quality assurance of        incoming data (i.e. maximum transit speed & previous known        position, only use ranges within a certain threshold)    -   The node updates its ‘known’ position, adjusts navigation        control as required and continues transit towards target        location using updated position        Acoustic Communication

USBL systems have limited number of unique beacon addresses (BAs)available for communication & positioning (this limit differs fordifferent USBL systems) and therefore have limited capacity to scale toa large number of nodes beyond this limit. This presents an issue whenattempting to position and communicate with a swarm of nodes severalorders of magnitude larger than the limit, as is required in embodimentsof the system 10 and method 100. To resolve this issue USBLcommunication and positioning must be managed using a non-standardapproach.

A proposed solution to this which may be incorporated in embodiments ofthe disclosed system 10 and method 100 is as follows.

Each node is each configured for a defined USBL beacon address (BA)prior to its deployment. This may occur at step 140 of the method 100.Respective groups of nodes are configured for (i.e. to have) the sameBAs. However, no two nodes in the same group with the same BA are ableto actively communicate over an associated USBL modem at the same time(i.e. during LANDING and TAKEOFF states).

To this end the node USBL transducer/modem 60 has a beacon functionalitywith two potential states ACTIVE and SLEEP. During the ACTIVE state thenode USBL transducer/modem 60 responds to ranging and communication andacts on information received. During the SLEEP state the node USBLtransducer/modem 60 is still receiving however does not respond toranging or communication and does not act on information received. Totransition from SLEEP to ACTIVE a wakeup message is sent from thetopside USBL system (i.e. from the USV 88 which may initially receivethis command from the CCS 98 by way of the surface communicationsnetwork 92) to the relevant node USBL beacon (modem) address.Additionally, node USBL BA can be configured remotely via acousticcommunications as required.

In one embodiment the acoustic communications may be realised byadoption of various strategies with a general aim of producing bandwidthrequirements which in broad terms may include the followings:

-   -   1. Prior to the start of a survey, defining and storing all        potential node target locations in an electronic table. In cases        where an operator may want to alter a node target positions        during execution of survey all possible/potential locations        could be defined. Here each location is assigned an ID, latitude        & longitude. If accurate depth at the target locations is        available it is included in table, otherwise this is left blank.        Before deployment of a node, while it is being configured in        step 140 (see FIG. 12) and in a CONFIG state, the entire table        of information may be parsed to each node. This allows target        locations to be identified using HEX id (i.e. a standard        hexadecimal character string) during survey operations,        significantly reducing acoustic communication data transfer        requirements.    -   2. The node target location table may also be used to define        commanded recovery locations during INITIATE_RECOVERY        transition.    -   3. Transitions and states IDs may be defined by ID. For example,        the node states described in the above Table may be defined as        corresponding states ST #, i.e. state ID1 may be defined as        state ST01. The node transitions from one state to another may        be defined as TR #, i.e. with reference to the state diagram in        FIG. 9, the node transition from TAKEOFF to RECOVERY may be        defined as TR16.    -   4. All target OWTT beacon location ID & metadata (source time        schedule, beacon target location etc.) may be defined prior to        survey start and parsed to at least a subset of, but preferably        all the nodes during CONFIG state. This data is stored on data        storage and/or databases of or otherwise accessible by the        controller 32. This information is utilised by transiting nodes        in the processing of received OWTT acoustic packets to calculate        position.        Method of Acquiring Data

With reference to FIG. 12 the method 100 involves a plurality of processor steps which may be broadly grouped into a pre-deployment process 128,a node deployment process 130, survey and data acquisition process 132,and a node storage and data download process 134.

Node Pre-Deployment

The pre-deployment process 128 involves an initial step 136 of deployingand positioning the vessels required in conducting the survey. In thisembodiment the vessels comprise the USVs 88, the source vessel 12 andthe NMS 90. In this embodiment two USVs, 88 a and 88 b (referred tocollectively as USVs 88) are used in conducting the seismic survey. TheUSVs 88 may be carried by and launched from the NMS 90. The USVs 88navigate to and are held at their respective start locations. It isduring this phase that all surface communication checks are performedthe CCS 98 and via the communications network 92.

At step 138 an automated pre-survey functional check of the nodes N isperformed while the nodes N are on the NMS 90 and more particularly whenstill housed in their respective CNDS 96. The pre-survey functionalcheck is run from the CCS 98 on-board the NMS 90. A notificationindicator on a CCS console and on each dock 204 indicates whether a nodeN has passed or failed in its pre-survey functional check. During theproject process the status of the node is updated to PRE_CHECK.

A node N that has failed the automated remains in its respective bay formanual checking and troubleshooting at the next available opportunity.The state of the failed node is updated to PRECHECK_FAILED.

Next step 140 is performed on the nodes which passed the pre-check. Atstep 140 the state of the node is updated to CONFIG and the node isloaded with configuration data and survey information while in theirrespective containers on the NMS 90. The node is queued in a deploymentschedule and the state of the node is now updated to ready_to_deploy.This completes the pre-deployment process 128.

Node Deployment

The deployment process/step 130 can now commence. This involvesdeploying each of the nodes N in the ready_to_deploy state at step 142.On triggering of the deployment step 142 the nodes N are moved fromtheir respective CNDS 96 to the LARS 94 and deployed from the NMS 90.Once a node N is deployed it enters a DEPLOYED state. The deployment atstep 142 occurs while the NMS 90 is slowly moving through the nodetarget deployment/landing sites. That is, while the NMS 90 is in transitthe nodes N are being autonomously deployed at scheduled times topredetermined/programmed landing sites.

Immediately after a node N enters the DEPLOYED state it transitions toan INITIAL_TRANSIT state at step 144. The node N is now under water andmoving toward its initial target location. At this stage the buoyancysystem 16 is operating as described above in relation to FIGS. 7a-7d topropel the node N to its target location. Acoustic communication isestablished between the node N and a USV 88 (or the NMS 90) and the nodeN navigates toward its target location using ultrashort baseline (USBL)positioning and communications. When in the vicinity of its targetlocation the node N transitions to a LANDING state and subsequentlylands at this target location at step 146.

Upon landing, at step 148 a node N sends a verification signal to theUSV 88 that it has successfully landed at the target location. Thetarget landing location ID and the actual landing location, measured byUSBL topside unit on the USV 88, are recorded to a CCS database and thenode now enters the LANDED state.

The deployment process 130 is repeated until the entire array of nodes Nhave been deployed and verified as LANDED.

FIG. 13 depicts a snapshot in time of the deployment process 130. Herethe NMS 90 is transiting (i.e. sailing) over an array of target landinglocations for the nodes N. As the NMS 90 is transiting, nodes N arebeing deployed from the LARS 94. Nodes N which have been most recentlylaunched from the NMS 90 are shown as triangles and a transiting towardtheir respective target locations using dead reckoned navigation.

As represented in FIG. 13 during this process the nodes N fan out fromthe NMS 90. Eventually while transiting, the nodes N establish acousticcommunication with the USVs 88 and navigate toward their respectivetarget landing positions. These nodes N are represented by rectangles,being within the USBL communications envelope 119. The nodes N whichhave landed at their target locations and are now stationary and coupledto the seabed, are represented by black circles. During this process theUSVs 88 maintain a substantially constant spacing from each other andfrom the NMS 90.

When an initial swarm of nodes has been deployed and landed at theirtarget locations the deployment process 130 has been completed.

The survey and data acquisition process 132 can now commence.

Survey and Data Acquisition Process

The landed nodes N form a sensing array for seismic surveying. Prior tothe commencement of the survey, and as shown in FIG. 14, the USV 88 a islocated at a bottom of and at or slightly behind a trailing edge 164 ofthe node array so as to be a safe distance from the source vessel 12when the survey commences. The USV 88 b is located at a bottom of aleading edge 166 of the node array and is also outside of the shot area162. FIG. 14 also depicts the source vessel 12 approaching the nodearray with and each of its in general alignment with the trailing edge164.

In FIG. 14 all the nodes have landed at their target locations and arerepresented by either black circles or stars. The landed nodesrepresented as stars are those nodes which have been configured prior tolanding by the CCS 98 to act as OWTT beacons. With reference to FIG. 9,and the above Node State table these nodes are in the state ST12.

The number of landed nodes N is greater than the number of nodes N usedat any one time to acquire seismic signals related to the shots fired bythe source vessel 12. In the system 10 and method 100 while a group oflanded nodes is receiving seismic data, at step 152 other nodes whichhave previously received seismic data are repositioning themselves inthe survey area in order to acquire further seismic data from subsequentshots fired from the source vessel 12. This repositioning is generallyin the same manner as described above in relation to FIGS. 1-2. Therepositioning, firing, and acquisition of data is continuously repeateduntil the entire survey is complete.

In FIGS. 15 and 16 different nodes N are in different states and cantransition between states. In FIG. 15 (and FIGS. 14, 16 and 17) alltransiting nodes are designated by a triangle and all landed nodesacting as OWTT beacons are designated by a star.

Additionally, in FIG. 16 the nodes are represented as follows:

-   -   all nodes N are represented by an outer grey rectangle with an        inner symbol    -   all nodes in USBL communication with USV 88 a at the trailing        edge 164 have a square with a black perimeter and white interior        as their inner symbol and are designated as N_(TR)    -   all landed nodes within the shot area 162 have either an inner        black circle as the inner symbol and are designated as N_(L), or        have a star as their inner symbol and are designated as N_(LB);        the nodes N_(LB) are acting as OWTT beacons    -   all transiting nodes (i.e. in the TRANSIT state ST09,        repositioning between successive landed locations) have a        triangle as their inner symbol and are designated as N_(TB)    -   all nodes in the process of landing or already landed that are        outside of the shot area 162 and in USBL communication with the        USV 88 b at the leading edge 166 have a square with a black        perimeter and white interior as their inner symbol and are        designated as N_(LE)

At the commencement of the survey (i.e. at step 150) with both USVs 88maintained in positions a safe distance from the source vessel 12, thesource vessel 12 commences shooting seismic signals.

Once the shot vessel 12 has traversed beyond a defined offset distancefrom the trailing edge 164 of the array, commands are sent from the CCS98 to USV 88 a to trigger take off of a first node N along the trailingedge 164 and when doing so provides a location ID relating to the targetrepositioned landed location of that node N. When the multiple nodes Nare within the USBL acoustic communication range envelope 119 of the USV88 a this process can be conducted simultaneously for the multiple nodesN. This is the node repositioning step 152 which is depicted in FIGS. 15and 16.

The commands sent by the USV 88 a includes a WAKEUP command which setsthe USBL state of the node to ACTIVE (receiving, responding & acting onreceived acoustic commands) then commands the nodes N to change statefrom LANDED to TAKEOFF. The nodes send respective acknowledgementresponses to the USV 88 a which is communicated via the communicationnetwork 92 to and recorded by the CCS 98. The nodes move off the bottomof the seabed by operation of their buoyancy system 16, enter theTRANSIT state and begin transiting to their new locations, on enteringthe TRANSIT state the node sets its USBL state to SLEEP. The take-offdoes not involve use of the thruster 18. As the transiting nodes N_(TB)are repositioning the are navigated by way of OWTT navigation receivingacoustic communications packets from the landed nodes acting as beaconsN_(LB).

The USV 88 a continues along the trailing edge 164 of the array,repeating the command process for each landed node along the trailingedge 164. Thus, a new trailing edge is created along which the USV 88 areturns issuing the same commands as when travelling along the previoustrailing edge 164. This process continues for the entire survey phase toproduce the rolling array of nodes N previously described.

As previously described during the repositioning step 152 the nodesmaintain acoustic communications for the purposes of navigation via the“beacon nodes” N_(LB). The landed beacon nodes N_(LB) form an array ofOWTT beacons at any one time to facilitate navigate of the transitingnodes N_(TB) from the trailing edge 164 of the array to form a newleading edge 166 of the landed node array using synchronous clock OWTT.The array of beacon nodes is dynamic in terms of the overall survey,i.e. the array moves or rolls with the overall array of nodes.Transiting N_(TB) nodes that are outside the communication range of aUSV 88 are provided with navigation information via the beacon nodesN_(LB).

The propulsion for the nodes to transit between respective landedlocations during a survey is primarily provided by the buoyancy system16. As previously described this system operates to adjust the buoyancyof the node between a positive buoyancy and a negative buoyancy tofollow a transit path that may contain one or more cycles of rising anddiving within the transit envelope TE to traverse the distance betweensuccessive landed locations. It should be understood that this mayinclude a single transition between positive buoyancy and negativebuoyancy if adequate to traverse the required distance. Therepositioning between successive landed locations is done without thenodes needing to surface or otherwise be recovered and redeployed.

Once a transiting node N_(TB) is nearing the leading edge 166, the nodechanges its USBL state to ACTIVE (receiving, responding & acting onreceived acoustic commands), when the node is within range of the USV 88b, the USV 88 b sends a command to the node commanding the node tochange state from TRANIST to LANDING. USBL communication from USV88 bnow takes over to navigate the node to, and land at, its targetlocation. (Where multiple nodes N are within acoustic communicationrange of USV88 a this process can be conducted simultaneously.) Thelanding nodes are depicted as a node N_(LU) and land along, and form,the leading edge of the array. During landing the thruster 18 may beoperated to assist in manoeuvring the node to its target landedlocation. The buoyancy system 16 can be operated if required to adjustthe contact pressure of the node on the seabed to optimise mechanicalcoupling for receipt of seismic data.

Once verified by USV 88 b that a node has successfully landed at itstarget location, state of the node is updated to LANDED. This statusupdate is sent to CCS 98 together with the recorded actual landedlocation and subsequently recorded in the CCS 98 data base.

The USV 88 b continues along the leading edge of array, repeating thecommand process for each node before returning along next line. Thisprocess continues for entire survey execution phase 132 on a schedulecontrolled by the CCS 98.

At a predetermined time before the firing of the last survey shot, arecovery step 154 begins in which the nodes N are recovered back ontothe NMS 90. In broad and general terms in this process nodes N whichhave received data from their last associated survey shot i.e. areoutside of the shot offset area, commence their journey to a collectionpoint to be recovered by the NMS 90. This process continues until allthe nodes N have been recovered. This will generally be some time afterthe source vessel 12 has at step 156 fired its last seismic signal.

FIG. 17 illustrates a snapshot in time during the recovery step 154. Inthis Figure unoccupied node positions P are represented as plain circleswith white interiors. The unoccupied node positions P are thosepositions previously occupied by the transiting nodes N_(TB). During therecovery phase the USVs 88 move to be in line with landed nodes tocommand and provide navigation information to the nodes. The recoverystep 154 is described in greater detail below.

Once the source vessel 12 has moved beyond a predetermined offsetdistance from a trailing edge 164 of the array of landed nodesN_(L)/N_(LB), the USV 88 b moves to join USV 88 a on the trailing edge164. Commands are sent by the CCS 98 to the USVs 88 via network 92 totrigger recovery of the first in line landed nodes N_(L)/N_(LB) andprovide ‘recovery location id’ of a target recovery location 168 to thenodes N_(L)/N_(LB). A ‘recovery batch id’ is also provided to the landednodes N_(L). Further where multiple landed nodes N_(L)/N_(LB) are withinacoustic communications range 119 of a USV 88 this process can beconducted sequentially.

This recovery command process may be timed in a way that all nodes ineach batch reach the target recovery location 168 at around the sametime. This can be achieved by arranging the USVs 88 to move in towards acentre of the line of landed nodes N_(L)/N_(LB) so that the outsidenodes N_(L)/N_(LB) are triggered to move the first. It is the differencein the travel distance to the recovery location 168 for the outer nodesN_(L)/N_(LB) in a particular line in comparison to that of the morecentrally located nodes N_(L)/N_(LB) that results in the nodes reachingthe recovery location 168 around the same time.

During this operation the location of the USVs 88 relative to the noderecovery locations is managed to avoid collision. This includes settingthe recovery location 168 to ‘behind’ trailing edge 164 of the array ofnodes so that the transiting nodes N_(T) move away from the USVs 88.

Batches of nodes come to surface near the recovery location and navigateon the water surface using GPS to target recovery location 168. Thenodes congregating as a batch at the location 168. The locations of thesurface nodes are monitored by the CCS 98 and once the nodes havecongregated, the NMS 90 moves into a recovery position.

The LARS 94 is activated while the NMS 90 holds its position. Theorientation of the NMS 90 relative to the batch of surface nodes atlocation 168 will be dependent on wind, wave & surface currentconditions.

The batch of nodes at the water surface at the location 168 begin tomove into LARS 94. During this process the nodes may maintain, orotherwise apply, thrust using their thrusters 18 to assist in engagementwith and/or collecting by the LARS subsystem.

The survey process 132 may also include two optional steps 158 and 160.At step 158 a random number of nodes N may be recovered for qualitycontrol purposes. The recovered nodes have their data checked andanalysed to provide a degree of confidence that the acquired seismicdata is in accordance with expectations. The nodes to be recovered maybe triggered to move from the trailing edge 164 of the array by a signalsent from the CCS 98 to a USV 88 and then to the nodes in question. Thenodes congregate at a recovery zone in a manner similar to thatdescribed above in relation to the recovery at the end of the survey andare collected by the LARS 94.

The recovered nodes are loaded into the docks 204 of the CNDSs 96, datais offloaded, and the quality control check on the data performed. Thisopportunity may also be taken to recharge the batteries 34 of the node.Functional pre-checks of the node may be performed similar to thatdescribed above in relation to step 138. Assuming the node passes thepre-check, its state is updated to ready_to_deploy the node is thendeployed ahead of the leading edge of the array. Subsequently the nodeis navigated to its target landing position by USV 88 b using USBLcommunications/navigation.

At step 160, in cases where survey duration exceeds the node endurance,a mid-survey node battery recharge and data offload can be performed.Scheduling of these operations is controlled by the CCS 98 to minimiseadditional node requirement. The recovery and redeploy of these nodesmay begin well before maximum node endurance is reached. This process issimilar to that described above in relation to the quality control checkexcept that offloading of data is not a critical function although thedata can of course be optionally performed if desired.

In the event that the number of nodes may require recharging during asurvey is about the same as used to perform the quality control check,the two steps 158 and 160 may be merged into one.

Node Storage and Data Recovery

Once the nodes are recovered onto the NMS 90 by the LARS 94 the nodestorage and data recovery process 134 (shown in FIG. 12) commences. Theprocess 134 involves three steps 165, 167 and 169. During the process134 the nodes are in communication with and controlled by the CC 98. Atstep 165 the LARS 94 is operated to return the nodes to an availabledocking bay in a corresponding CNDS 96 as previously described withreference to FIG. 11 b.

Next at step 167 the data from the nodes acquired via their sensors 76,is offloaded via the data offload. This may be via the connector 80, orWi-Fi or by optical communications. Lastly at step 169 post-surveychecks may be made of the nodes to verify their functionality andsuitability for redeployment. The node is now powered down, i.e. turnedOFF. This process is repeated for each batch of nodes until all thenodes in the array are recovered and in their container docking bay.

Second Embodiment—Ocean Survey Application

The first embodiment described above relating to seismic surveys ischaracterised as such by the provision of seismic sensors in the nodes Nand the transiting nature of the nodes between multiple landed locationswithout surfacing to provide rolling or dynamic bottom coupled seismicsensor array. Communications and navigation systems are incorporated inthe first embodiment to enable the dynamic nature of the array that mayincorporate hundreds or thousands of nodes and are required toaccurately reposition themselves to specific target locations tens orhundreds of times over a several months period.

The second embodiment is in general terms a remotely operated,scaled-down version of the first embodiment, utilising a subset of thefull system capability and is envisaged for use in remote ocean surveys.In an ocean survey application, it is envisaged that the swarm of nodeswill transit a significant distance (10's to 100's of kilometres) from adeployment location to a survey location, primarily using buoyancypropulsion, periodically surfacing to obtain a GPS fix, to send a statusmessage via satellite communication to an onshore base and to receiveany command updates. Then upon reaching the target site, they will divedown to seabed, and optionally perform a landing manoeuvre and couplewith the seabed as described in the first embodiment, holding positionon the seabed. Nodes will remain landed until either a trigger caused bythe passage of time or in response to information collected by anonboard sensor (and associated data processing systems). In someembodiments, some of the required ocean data may be acquired while thenode is above the seabed and thus where the node does not land; orindeed while the node is in transit.

The system and method associated with the second embodiment may involvethe use of a relatively small number of nodes for example 10-20 ratherthan the hundreds or thousands of the first embodiment. Also, positionalaccuracy may not be as important as in the first embodiment to thequality of recorded data.

The nodes may operate completely independently of any surface vessel(i.e. deployed from and retrieved to an onshore location); or, they maybe supported by one (or more) long duration USVs 88 or other surfacevessel for positioning and acoustic communication.

The nodes may remain on or at a first site for the entire period of thesurvey; or, move between several survey sites. These sites may be asignificant distance apart such as for example 10s to 100s km, at whichthe above process is repeated.

For these reasons the second embodiment does not require some of thecomplexity in terms of acoustic communications and navigation of thefirst embodiment. The substantive differences between the first andsecond embodiments is summarised below.

Nodes

-   -   The first embodiment describes a buoyancy system having a pump        system 28 with either separate transit and lift-off pumps which        operated different pressures and flowrates, or a single pump. In        the second embodiment it is envisaged that the nodes only        require a single pump, although the node will still of course        operate with a dual pump system.    -   Instead of or in addition to the seismic sensors 76 a, the        sensors 76 b are carried by the node N. As previously mentioned        the sensors 76 b include but are not limited to sensors for        sensing or measuring characteristics or physical attributes such        as but not limited to: water temperature, density, chemical        characteristics such as salinity, pH, oxygen content, carbon        dioxide content, phosphate content; sulphur content;        oceanographic conditions including ocean current velocity and        hydrostatic pressure; gravitational field strength; magnetic        field strength and orientation; gamma radiation; acoustic        characteristics; optical characteristics; bathymetry; and        aspects of the benthic zone. The node may also include still        and/or video cameras.

System

The system of the second embodiment may differ from that described inrelation to the first embodiment as follows:

-   -   The source vessel 12 is not required as seismic data is not        being acquired.    -   A node mothership or other surface vessel may or may not be        required for node deployment, recovery, maintenance and        transport to required measurement locations. This will be        dependent on the task at hand and take into account factors such        as whether: the sensing locations are within the travel range of        the nodes from an onshore base; and/or the projected time for        conducting the survey would exceed the endurance of the nodes if        operated from an onshore base. In the event that a node        mothership or other support vessel is needed it is unlikely to        require the infrastructure of the NMS 90. For example, a single        and/or scaled down version of the CNDS 96 would be sufficient        for the small number of nodes required. Indeed, it may not        require a CNDS at all. Instead of the nodes could be stored in        cradles on deck, or in a cabinet and manually deployed.    -   When a mothership or other support vessel is used for deployment        of the nodes, it may then return to shore leaving the nodes to        acquire their data. Once the data has been acquired a mothership        or of the support vessel can make a further trip to recover the        nodes.

Method

The method of acquiring data in the second embodiment is simplified incomparison to that of the first embodiment as the nodes will operate ina scaled down swarm size (1 s-10 s) and be controlledremotely/autonomously.

The steps in one embodiment of the method may entail the following:

-   -   if a launching support vessel (e.g. mothership or USV) is used        the nodes may be configured while on the support vessel as in        the first embodiment;    -   launching the nodes;    -   enabling the nodes to autonomously fly to a designated target        location;    -   acquiring the relevant ocean survey data at the measurement        location;    -   triggering the nodes (for example based on effluxion of time or        some other trigger signal) to surface for reposition, remote        data transfer or recovery.    -   reposition to next location (or repeat landing at first        location) etc until completion of survey.

In performing the method, the nodes may use their OWTT capability toposition relative to each other. Also, data or other information can betransferred between nodes in the swarm using their acoustic modem 60.

FIGS. 18a and 18b illustrate aspects of the second embodiment.

FIG. 18a shows an example of a round-trip for a node used for an oceansurvey. One or more nodes may be carried to a deployment site 300 on forexample a USV or another surface vessel. The nodes are deployed at thesite 300 and may travel using their buoyancy propulsion system describedabove in relation to the first embodiment to a first survey site 302.The first survey site 302 may be up to tens or hundreds of kilometresaway from the site 300. At the site 302 the nodes land and acquire thedesired ocean data using the sensors 76 b.

The nodes N subsequently reposition to a second and a third survey site304, 306 respectively to acquire ocean data. Once ocean data has beenacquired from the designated sites the node(s) transit to a recoverysite 308. The recovery site and node may be the same as or different tothe deployment site 300.

As previously mentioned, the nodes N are triggered to move betweenvarious sites by way of a trigger signal, such as the effluxion of time.Between moving from site to site the nodes may surface to acquire GPSdata to assist in navigating to a subsequent site. If and when surfaced,one or more of the nodes may also transfer data for example viasatellite to a control centre. In this regard data from a plurality ofthe nodes may be transferred to a designated data transfer node whichtransfers all the data from all the nodes when surfaced.

When the second embodiment incorporates a plurality of nodes the nodesmay navigate without surfacing between survey sites using one-waytransit time positioning as described above in relation to the firstembodiment.

As shown in FIG. 18b the buoyancy of a transiting node may be controlledor modulated by the buoyancy system 16 in the same manner as describedin relation to the first embodiment causing the node to traverse withina transit envelope between successive landed locations. However, in adeparture from the seismic survey embodiment, in order to updatedposition data and transfer acquired data the transit path may includeone or more nodes surfacing for a period of time.

The approach to landing and control of buoyancy during this process isthe same as shown in and described with in relation to FIG. 7 d.

Now that embodiments of the nodes, system and method have been describedit should be appreciated that the nodes, system and method may beembodied in many other forms.

For example, the array formed by the landed nodes N need not berectangular and can include other shapes irrespective of their abilityto tessellate. Moreover, the array shape and pattern can change fromlocation to location to account for bathymetry and/or structures (e.g.platforms, jackets, pipelines etc) disposed in a survey area.

In the example described above in FIGS. 1a and 1b of the dynamic rollingnature of the arrays, the nodes are shown as moving to “like for like”locations from one array to the next. However, this is for ease ofdescription only and it is not essential for such concordance in nodelocation. As the nodes are dynamically repositionable they may takedifferent relative positions from one array to another. This will be thecase when one array covers an area of different size or shape ofanother, or use a different number of nodes. Also, as mentioned abovethe system 10 may comprise a plurality of backup, redundant or reservenodes which may transit from location to location but are not activatedto provide readings of physical characteristics or parameters unless anduntil called upon to take the place of a node which has become disabled,or augment measurement/reading density. Additionally, or alternately,further nodes may be carried by the command vehicle and deployed to joina current swarm of nodes.

Embodiments of the nodes may also be arranged to enable data transferbetween each other. In this way several nodes may communicate withanother node that comes to the surface to act as a gateway for transferof data to a surface vessel or shore-based system.

In the claims which follow and in the preceding description, exceptwhere the context requires otherwise due to express language ornecessary implication, the word “comprise” and variations such as“comprises” or “comprising” are used in an inclusive sense, i.e. tospecify the presence of the stated features but not to preclude thepresence or addition of further features of the nodes, system and methodas disclosed herein.

What is claimed is:
 1. A submersible autonomous data acquisition nodecomprising a buoyancy system enabling variation of the node buoyancybetween positive buoyancy and negative buoyancy, the buoyancy systemincluding: at least one inflatable external bladder; at least oneinternal reservoir containing a liquid; a pump system having at leastone pump operable to transfer liquid from the reservoir to the at leastone inflatable external bladder to thereby increase total node buoyancy;a bleed path operable to selectively allow the liquid to flow from theat least one inflatable external bladder to the reservoir by action of adifference in pressure acting on liquid in the reservoir and in the atleast one inflatable external bladder, to thereby decrease total nodebuoyancy; and a controller to control the pump system and the bleed pathto achieve a desired buoyancy, wherein the controller is configured tovary node buoyancy between a positive buoyancy and a negative buoyancyduring transit between first and second landed submarine locations onthe seabed so that the buoyancy system is operable as a buoyancypropulsion system, and wherein the controller is configured upon thenode landing at a seabed location to increase the negative buoyancy ofthe node beyond that required to dive to the seabed location to therebyincrease contact pressure between the node and the seabed.
 2. The nodeaccording to claim 1, wherein the controller is arranged to vary nodebuoyancy between a positive buoyancy and a negative buoyancy to maintainthe node within a transit envelope below the water surface and above aseabed during a transit phase for a node traversing from a firstsubmarine location to a second submarine location.
 3. The node accordingto claim 1, wherein the controller is operable to vary node buoyancybetween the positive and negative buoyancy in a manner wherein a nodetraverses within the transit envelope along an oscillating path.
 4. Thenode according to claim 3, wherein the node is configured when landed onthe seabed in manner wherein the negative buoyancy of the node issubstantially evenly distributed over contact locations between the nodeand the seabed.
 5. The node according to claim 1, wherein the reservoirincludes a pneumatic region separated from a hydraulic region containingthe liquid, and wherein the pneumatic region is at a negative pressurewith respect to atmospheric or hydrostatic pressure acting on the atleast one inflatable external bladder.
 6. The node according to claim 1,wherein the bleed path includes a one-way valve operable to enable fluidto flow only in a direction from the at least one external inflatablebladder to the reservoir, the valve being switchable between an openedstate wherein liquid is able to flow from the at least one externalinflatable bladder to the reservoir, and a closed state in which theliquid is blocked from flowing through the bleed path.
 7. The nodeaccording to claim 1, wherein the pump system comprises a single pumpcapable of pumping liquid from the reservoir to the at least oneinflatable external bladder.
 8. The node according to claim 1, whereinthe pump system comprises a transit pump and a lift-off pump, whereinthe transit pump is arranged to pump liquid between the reservoir andthe bladder at a first flow rate and first pressure, and the landerbuoyancy system comprises a lander pump which pumps liquid between thereservoir and the bladder and a second flow rate and second pressurewherein the first flow rate is higher than the second flow rate and thesecond pressure is higher than the first pressure.
 9. The node accordingto claim 1, wherein the node is selectively operable to broadcastacoustic communications packets and to receive and process acousticcommunications packets to facilitate one-way travel time positioningwhen the node is repositioning between two submarine locations.
 10. Thenode according to claim 9, wherein the node comprises a top mountedtransducer for broadcasting the acoustic communications packets and adownward facing receiver for receiving acoustic communications packetstransmitted by another node.
 11. The node according to claim 1, whereinthe node is arranged to communicate with a surface marine vessel usingUSBL, SBL or SSBL acoustic communications.
 12. The node according toclaim 1, further comprising a surface communications system enabling anode to communicate when on a water surface with the or another surfacevessel and/or a land-based station.
 13. The node according to claim 1,further comprising a GPS receiver.
 14. The node according to claim 1,further comprising a thruster arranged to automatically operate whenspeed of a node derived from operation of the buoyancy propulsion systemis less than a threshold speed.
 15. The node according to claim 1,further comprising one or more seismic sensors supported on the node foracquiring seismic data.
 16. The node according to claim 15, furthercomprising one or more other sensors.
 17. The node according to claim 1,further comprising one or more sensors capable of sensing one or moreoceanographic characteristics or properties.
 18. A marine dataacquisition system comprising a plurality of nodes: wherein each node ofthe plurality of nodes comprises: a buoyancy system enabling variationof the node buoyancy between positive buoyancy and negative buoyancy,the buoyancy system including: at least one inflatable external bladder;at least one internal reservoir containing a liquid, a pump systemhaving at least one pump operable to transfer liquid from the reservoirto the at least one inflatable external bladder to thereby increasetotal node buoyancy; a bleed path operable to selectively allow theliquid to flow from the at least one inflatable external bladder to thereservoir by action of a difference in pressure acting on liquid in thereservoir and in the at least one inflatable external bladder, tothereby decrease total node buoyancy; and a controller to control thepump system and the bleed path to achieve a desired buoyancy, andwherein the controller is configured to vary node buoyancy between apositive buoyancy and a negative buoyancy during transit between firstand second landed submarine locations on the seabed so that the buoyancysystem is operable as a buoyancy propulsion system, and wherein thecontroller is configured upon the node landing at a seabed location toincrease the negative buoyancy of the node beyond that required to diveto the seabed location to thereby increase contact pressure between thenode and the seabed.